Para-dichlorobenzene is frequently produced by chlorinating benzene and/or chlorobenzene. The formation of para-dichlorobenzene is accompanied by the formation of various by-products such as ortho-dichlorobenzene, meta-dichlorobenzene, more highly chlorinated benzenes, and often some degredation products. In many instances once the desired amount of para-dichlorobenzene (usually, but not necessarily close to the maximum yield attainable) has been produced, the chlorination is terminated in order to conserve chlorinating agent and to maintain the production of undesirable higher chlorinated benzenes at acceptably low levels. It is usually found in these situations that the principal by-product is ortho-dichlorobenzene.
One known way to favor the production of para-dichlorobenzene while reducing the coproduction of the less desired ortho-dichlorobenzene, is to conduct the chlorination in the presence of a catalytic amount of zeolite L catalyst. Such chlorinations are discussed in U.S. Pat. Nos. 4,724,269 and 4,777,305; in European Patent Application Publications No. 0 118 851, 0 154 236, and 0 171 265; and in Huizinga et al, Tetrahedron Letters, Volume 21, pages 3809-3812 (1980), the disclosures of which are, in their entireties, incorporated herein by reference, including especially their descriptions of zeolite L catalysts, the making of such catalysts, and the use of such catalysts in the chlorination of benzene and/or chlorobenzene. U.S. Pat. No. 3,216,789 discloses zeolite L, its preparation and characteristics. D. W. Breck, Zeolite Molecular Sieves, pages 156 and 361, and L. Wilkosz, Przemys Chemiczny, Volume 51, No. 8, pages 524-527 (1972) discuss zeolite L and its characteristics. The disclosures of U.S. Pat. No. 3,216,789 and the cited pages of the Breck and Wilkosz publications are, in their entireties, incorporated herein by reference.
A convenient ratio for characterizing the relative proportions of para-dichlorobenzene and ortho-dichlorobenzene in the organic feedstock, in organic intermediates, and in the organic reaction product is the "para/ortho ratio", often abbreviated as "P/O" or "P/O ratio", which is the ratio obtained by dividing the moles of paradichlorobenzene present by the moles of ortho-dichlorobenzene present. Sometimes the "ortho/para ratio", often abbreviated as "O/P" or as "O/P ratio", which is the inverse of the P/O ratio, is used for characterization purposes.
While the P/O ratio is useful for monitoring relative proportions of para-dichlorobenzene and ortho-dichlorobenzene, it provides little or no information as to how far the chlorination has progressed. A convenient scale of reference which provides this information is the "x-value" which reflects the degree of chlorination of the benzene-based compounds in the organic feedstock, in organic intermediates as the reaction progresses, and in the organic reaction product. The x-value is the value of x in the empirical formula for a mixture of such compounds: C.sub.6 H.sub.6-x Cl.sub.x. It may be calculated as follows: ##EQU1## where A=moles of benzene present:
B=moles of monochlorobenzene present; PA1 C=moles of dichlorobenzene (all isomers) present; PA1 D=moles of trichlorobenzene (all isomers) present; PA1 E=moles of tetrachlorobenzene (all isomers) present; PA1 F=moles of pentachlorobenzene present; and PA1 G=moles of hexachlorobenzene present.
The x-value of the organic reaction product is greater than that of the organic feedstock.
Although zeolite L favors high P/O ratios for organic intermediates and organic reaction products, its effectiveness in this regard declines with continued use. Thus under substantially similar reaction conditions using substantially similar organic feedstocks, the P/O ratio at a given x-value is often lower for later chlorinations than it is for earlier chlorinations using the same zeolite L catalyst. Eventually the catalytic effectiveness of the zeolite L becomes nil.